Session 4. Project Discussion

I will use this session space to address a number of issues relating to course protocols, the project task, and to information that relates to the responses posted to date in the Discussion section of this course site.

Projects:

With respect to the Project, the postings so far indicate the identification of a number of excellent topics and case studies that can be explored. As I stated when we first met, the projects and topics will be developed by groups of three students. I've tried to look through people's backgrounds and introductions and put them together in groups that currently make some sense:

Group 1: Diana Esshaki, Dina Katz, and Marcus McLean

Group 2: Elizabeth Antonellis, Eric Faulkner, and Despina Felis

Group 3: Michael George, Lisa McIntosh,and Erin Valentine

Individuals in each of these groups should contact one another and look over the topics posted by the other members of their group along with the arguments as to why these topics and case studies are important. By March 7 each group should have settled on a topic and a set of no more than two related case studies that the group will develop into an .html/Web document presentation for evaluation by other members of this class and by the instructor. The product needs to include a description of why this topic (or these case studies) are of importance, the pertinent facts in the case studies, how they relate to one another, where toxicology plays a role, and what that role is. The project must present the conclusions of the student authors about synthesis and integration of the information gathered regarding compounds or case studies with respect to use of toxicological approaches, risk assessment, and whether any resolution of the problems involved were actually found. Thus, though the HTML document may include some lists or tables, it should go beyond this to make significant comments about the information that has been gathered. There needs to be an analysis done and presented that covers the chosen project topic. Finally, there must be a clear indication of what each student's contributions were to the project.

By mid-April a penultimate (next to final) version of the project should be posted to the Discussion area I set up for the purpose. Students from the other groups should read through each of the other projects and post their responses and suggestions as replies over the next week. Then the originating group should review the comments and make their own final adjustments and modify the presentation accordingly. The final presentation should be posted to the Prometheus site in the appropriate Discussion area by the second week of May.

Discussions to Date:

Nearly all responses to questions or topics posted to date have been directed to the instructor. A number of people have complained that the questions posted were vague. Please note that several of these have been purposely vague because they are attempts to get a discussion started. It is quite reasonable to think that many of these questions would require a long essay to address them adequately. Indeed, a book might be required! What is needed from students in such cases is a start, a beginning. The statement of a fact and some thoughts on a subject which can then be followed and built upon by others. So far, there are too many restatements of what appears in the text rather than actual thoughts concerning a question. In what follows, I will try to flesh out a bit of what I hoped might appear in some of those discussions as well as some additional pieces of information you might find useful or helpful in thinking about mammalian toxicology. I do not disagree with most of what has been posted so far, I simply want to amplify a few points.

Causality:

Subject: Question Set 1. Question 1. Questions arising from the historical background of toxicology.

1. The idea of causality arose during the middle ages and Renaissance. This builds on careful observation and testing. How might this give rise to the notions of occupational hazards or the nonmystical explanation for oncogenesis?

Our present concepts of causality arise out of the very Western notion of a cause being a necessary and sufficient condition for an effect, if A occurs then B must follow. So A is both necessary and sufficient for B to occur even though B might arise through some other route. Note that this does not say that if B occurs, A must be present, simply that B must follow if A is present. But these kinds of logical arguments really only were codified and formalized by the Greeks and others that followed. Thus, even if the idea that a plant, animal, or mineral would cause or cure a particular condition existed from the time of antiquity, the actual logical context wasn't present until relatively recently. The idea of scientific testing follows even later. That does not mean that ancient peoples did not associate jobs or exposures with deleterious outcomes, but the longer the gap between exposure and effect, the more unlikely the toxic impact would be discerned as something other than witchcraft or fate. The usual gap between exposure and cancer formation is long, minimally months to years. So unless a cancer was formed at the immediate site of exposure and that site was normally associated with the job involved, it would have been quite difficult (and still is) to argue that something at a job site caused a cancer. It is really only in the most egregious cases that this linkage is obvious. Most frequently case-control studies using one group in a suspect job and one matched for age and every other variable possible but occupation are needed to build an epidemiological case for a causal link between occupation and cancer formation. Even in animal studies, the tests often have to be done in a double-blind manner, in which one investigator codes the animals and the test or placebo materials and another administers them and evaluates the results before the first breaks the codes, in order to obtain statistically significant associations that can be used to build a case for causality. So it's not surprising it took a long time for specific cases of causality to be built with respect to cancer formation.

Biochemical Kinetics:

Subject: Question set 1. Question 2. Questions arising out of the historical background of toxicology.

2. What information already discussed is subsummed in the notion of biochemical kinetics?

Biochemical kinetics attempts to describe the dynamics of a biochemical system by describing the concentrations of various elements of that system at several points of time. The notions of dose, available dose, route of exposure, partitioning, and degradation (and/or toxification and detoxification) all fall within the realm of biochemical kinetics. You have already read, or will soon read, several lenghthy discussions on toxin transport and distribution in target organisms and systems. These topics are often described as toxicokinetics. As the models and concepts can also be used to describe movement and interactions of drugs or hormones within a biological system, we can also consider these areas as parts of biochemical kinetics.

Chronic Toxicity:

Subject: Question set 1. Question 6. Chronic Toxicity & Carcinogenicity

6. Note the emphasis of chronic toxicity testing and mutagenesis testing on carcinogenesis. Is this emphasis currently appropriate, or are there other possible chronic outcomes to be concerned with?

Much of toxicology has grown up in the past 150 years. It is often a good idea to keep in mind some of the historical changes that have occurred over that time period that might leave their mark on the science being done. By 1850 the idea that many diseases were caused by microorganisms was accepted. Vaccination was being used for smallpox and was being developed for other diseases by 1900. So infectious disease, though still devastatingly common, was becoming understood and was succumbing to sanitation practices and medical advances as toxicology developed. Genetic diseases were known and were beginning to be understood with respect to transmission by 1920. That left chronic and metastatic diseases as the unexplained killers of most of the human population. And that situation persisted until at least the 1950s when medical advances began to provide patients with some hope of long term survival or cure for these conditions. But note that cancer was "the" big unexplained threat to life through much of the period when toxicology and the tests associated with it were being developed. While chronic autoimmune conditions like rheumatoid arthritis were known, they usually didn't kill people outright. But cancer did. Senile dementias were of lesser importance because large numbers of people did not live long enough to develop them. Thus, toxicity testing developed in a political and medical environment that emphasized the importance of cancer and deemphasized the importance of chronic immune, endocrine, and CNS deficits. Some of that is now changing as we have begun to understand more about chronic diseases and their possible induction by toxicant insult. Likewise emphasis is also being placed on genetic, reproductive, and developmental insults in toxicology studies. Carcinogenicity will always remain an important part of toxicology, but the dominating role it has played in the past will probably decline as the importance of insults to other systems becomes more apparent.

Metabolism & Development:

Subject: Question Set 1. Question 7. Toxin Metabolism Considerations

7. Emphasis in the discussion of biochemical and physiological means of toxin metabolism and clearance is on the adult animal. Are there other important considerations or routes to be considered in growing offspring or during the reproductive process?

This topic will occupy several sessions during which reproduction and development will be considered. The big differences here concern the relatively robust defense mechanisms available to the adult organism for detoxifying, clearing, and partitioning toxicants to decrease their bioavailability at the target sites versus those available in gametes, the zygote, developing embryo, fetus, and newborn offspring. There are differences in the repair systems and in the impact of toxicant action on the organism if intoxication kills a few hundred or thousand cells. Obviously, if the organism only has that many cells, killing them is lethal, whereas in the adult it may only be a minor discomfort. There are also big differences in physiological functions. For example, in the fetus and newborn in many mammals the primary function for the liver is not central metabolism but hematopoiesis, it is not making glucose so much as it is making red blood cells loaded with fetal hemoglobin. And fetal hemoglobin has a higher affinity for oxygen and any other heme ligands than adult hemoglogin does. Similarly, the fetal adrenal cortex synthesizes and processes steroids in a much different way than the adult adrenal cortex does. As a result of these differences, the early stages of organismal development present a different array of target tissues as well as differences in tissue function that make significant differences in how toxins may reach, interact with, and be cleared by organisms in those stages.

Risk & Toxicity Levels:

Subject: Question Set 1. Question 10. Risk Strategies & Levels of Toxicity

10. What distinctions might be made between lethality, morbidity, reproductive capacity, quality of life, and quality of environment impacts vis a vis risk assessment strategies?

What can be said about toxicity? A toxin can have a lethal or a sublethal effect. So, is one word adequate to define toxicity? It would seem there is a need to define varying types and levels of toxicity or toxic response.

Lethality: mortality, death (induced reasonably rapidly as an obvious consequence of exposure)

Morbidity: a decrease in functionality that may ultimately lead to a decrease in lifespan

Reduced reproductive capacity: decrease in reproductive output (this has a genetic carry-over to the next generation because there is a resulting change in the potential gene pool)

Decrease in specific functions or acuities (sensory: sight, hearing, taste, smell, touch; specific metabolic functions - food allergies; appearance - hair growth, acne): these decrease quality of life but are often nonlethal, do not decrease lifespan and do not alter the gene

Reduced asthetics or other quality of life issues: these are often issues that have little or no medical impacts except at the psychological or emotional levels

Reduced quality of the environment: this may impact any or all of the above items

Note that these range from severe immediate impacts on the individual, through prolonged or delayed impacts on the individual or the species, to subtle effects that may only alter survival or functions of individuals or species via alterations of behavior or mental vitality. Evaluating risks of toxic exposure then may encompass analysis of biological risks and exposure tests, potential medical or economic risks of exposure to or lack of exposure to the toxicant of interest, and possible impacts on the integrity or improvement of environmental quality. The overall picture may involve a variety of competing interests or benefits and definitely involves a cost/benefit analysis. Even at the individual level this is reflected in the statements that exist on drug and household chemical packaging. For medications there are not only directions for proper use, but indications of possible deleterious side effects. These are often unrelated to the drug target tissue of interest, e.g., antihistamines not only decrease swelling in nasal mucus membranes, they often suppress mental alertness and/or increase blood pressure. The detailed analysis of positive and negative drug impacts is subsumed in the science of pharmacology but has obvious overlap and relation to toxicology both in its tenets and methodologies but also in the subjects of study and the analyses used to evaluate study results.

Oral Toxicity Paradox:

Subject: Question Set 2. Question 6. Oral Toxicity Paradox

6. Oral toxicity studies have defined a paradox of this route of exposure. What is the paradox and how might it be explained? A somewhat similar paradox can arise if dietary insoluble fiber is high and exposure is again oral. Why might this occur? Are either of these situations of possible use in treating acute poisonings? In chronic intoxications?

No one really seemed to get the factual part of this one. Check the bottom of page 113 and the top of 114 in C&D: "the oral toxicity of some chemicals is increased by diluting the dose..." So here, a decreased concentration increases toxicity! This is "explained by more rapid stomach emptying induced by increased dosage volume, which in turn leads to more rapid absorption in the duodenum because of the larger surface area there." The paradox of toxic intake by the oral route is that lower dose concentrations (and larger volumes) can sometimes be more toxic than higher dose concentrations [same amount of toxicant in either case, just different volumes].

Note the volume sensitive sphincter separating the stomach and the small intestine which has a very large absorptive layer. If the volume of the dose is increased this may lead to a faster emptying of the stomach, a lesser time for the HCl in the stomach to potentially inactivate the drug or toxicant, and more rapid exposure of the intestinal lining to a toxicant containing solution. In this situation even a lower total dose of compound may be more effective than a small concentrated dose.

Is there a secondary paradox? What if dietary insoluble fiber is present in the stomach at the same time as the dose? And what is insoluble fiber?

Insoluble fiber tends to be macromolecular and will include polymers that are nondigestible for the species in question, e.g. cellulose (in nonherbivores), hemicellulose, pectin, silicates, and some minerals.

An increase in insoluble fiber will increase the transit rate through the gut, i.e., it will decrease the time needed to transit the gut. This effect is often protective as it reduces time of exposure to a toxicant. The effect is also enhanced by the presence of clean insoluble fiber as opposed to previously contaminated fiber. This is because clean fiber can efficiently adsorb toxicants from solution and effectively sequester them from absorption by the intestinal lining.

But contaminated fiber can also release toxicants for absorption. So there is an equilibrium between toxicants free in solution and those particulates or colloids, including insoluble fiber, within the digestive tract. Insoluble fiber can act as a sink or source for toxicants depending on its prior exposure to toxicant.

Note that some of these effects will differ among species because of differences in gut physiology, a topic we will also cover in future sessions.

Radiation Exposure:

Subject: Question Set 3. Question 4. Time, dose, shielding.

4. In radiation protection there are three cardinal rules for minimizing radiation exposure:

1. minimize time of exposure to the source

2. maximize distance from the source to minimize dose delivered

3. optimize the amount and type of shielding used to minimize dose delivered.

Can these be translated into considerations in toxicant exposure? What are the analogs of time, dose, and shielding for chemical exposures? Are there any differences among routes of exposure or nature of toxicant that need to be considered?

Such questions raise the points of health concerns during radioisotope use, i.e., radiation health physics (RHP) concerns. RHP is concerned with minimizing the exposure of subjects to the electromagnetic and particulate emissions from radiation sources. Note that both of these types of radiation primarily impact targets or subjects by delivering energy: in the case of x-rays (arising from electron transitions in decaying radioisotopes, often a fall from a high energy outer orbital into a low energy inner orbital accompanied by the release of a corresponding photon of high energy electromagnetic radiation) or gamma rays (arising from transitions within the unstable nucleus, e.g., conversion of a neutron to a proton and a beta particle (energetic electron) often with the loss of a photon of high energy electromagnetic radiation) the energy delivered is in the form of high energy photons that can cause chemical bond alterations including breakage and/or molecular motions disruptive of normal molecular function (e.g., in biological systems). Particulate emissions like beta particles, positrons (positively charged electrons), alpha particles (2 neutrons + 2 protons), or neutrons can impart kinetic energy to target molecules causing either movement or bond disruptions. If particle bombardment is intense enough, the larger particles like alphas or neutrons, especially, can also cause nuclear alterations that can generate secondary radioactive atoms, e.g., chlorine. Thus, in either case energy is being imparted to the target and may result in alterations, some deleterious, to the target. Particles, however, in colliding or glancing off the electron shells of the atoms of the target, rapidly lose energy and, with the exception of the neutron, do not penetrate efficiently within a target medium. Neutrons can penetrate because they lack a net charge, they have no intrinsic tendency to approach or be repelled by the electrons, or protons, of target molecules, they are unaffected by the atomic magnetic fields and do not affect those particles or fields unless they undergo a direct collision with a nucleus or electron. (Photons penetrate better than charged particles but have an intrinsic electronic and magnetic character that causes them to interact with electrons and atomic magnetic fields; when the allowed energy transition levels of target molecules match those of the photon moving through the target medium, those photons are efficiently absorbed.)

Recapping the electromagnetic spectrum: long wavelengths mean fewer photons per unit time through a linear distance, lower frequency, lower energy (wavelength is inversely proportional to frequency. Radiowaves with wavelengths of >um (up to m or km) occupy the long end of the spectrum, followed by microwaves (um lengths), infrared (IR, heat, ~800 nm - a few um), visible (300-800 nm), ultraviolet (UV, 10's-300 nm), x- and gamma-rays (angstrom - nm), cosmic rays, and intranuclear forces (atomic/subatomic dimensions).

Note that since radiation is propagated from the source in all directions the flux of that radiation across any plane placed near the source will be inversely proportional to the square of the distance from the source (distribution in two dimensions, not just one). Thus, several of the rules of RHP have to do with maximizing the distance or effective distance from the source.

The three cardinal rules of RHP are:

1. Minimize time of exposure

2. Maximize distance from source

3. Optimize shielding (protective clothes, gloves, shields, behaviors)

Note that 3. is actually a variant of 2. in that it addresses "effective" distance from the source. Thus, if a radiation absorbing shield is interposed between the source and the subject, we are essentially increasing the effective distance between the source and the subject. Note also the indication of optimizing shielding. Although it might be intuitive to use a high density shield which normally corresponds to a shield made of a material of high atomic number, we should be aware that such materials provide larger atomic targets for some forms of radiation, particularly highly energetic particles. When such highly energetic particles, e.g., the beta particle from 32P decay, encounter large atoms (e.g., in lead or leaded glass shields) they often cause electron orbital alterations that result in the production of secondary particle (Compton electrons) or x-ray emission (Bremstrallung). As the electromagnetic radiation is more penetrating than most particles, the secondary emissions may prove more damaging than the primary radiation. Thus, in such cases, low to moderate sized atomic nuclei are used to build composites that can then be used as effective shielding, e.g., high density plastic (carbon/oxygen/hydrogen) or aluminum.

Now note that these same rules for limiting radiation exposure have close or exact analogs in rules to limit chemical or toxicant exposures:

1. Minimize time of exposure. (This limits the dose and/or the time for the toxicant to bind to its target molecules or to act on its target molecules.)

2. Maximize distance from source. (This minimizes the dose or encountered concentration.)

3. Optimize shielding. (This limits the routes of exposure and/or optimizes the actions of endogenous barriers, e.g., the immune system -- vaccines, or repair processes.)

So the chemical protection rules for toxic substances are virtually identical to those for exposure to radioactive sources. And while some radioactive isotopes are also chemical toxins such as Cesium or Plutonium, the usual intoxication for these materials is absorption of too much energy from the radioactive decay products.

Prevention & Treatment of Intoxication:

So how do we go about preventing or treating toxicity?

For the individual:

1. Avoid exposure

2. Minimize dosage

3. Maximize elimination (e.g., modify urine pH, often alkalinize)

4. Block circulation and absorption of toxicant (chelators, antibodies, charcoal, clays)

5. Apply antagonists (for specific molecular targets)

6. Promote repair

For the environment:

1. Modify chemical use and disposal

2. Institute regulations

3. Minimize releases or exposures

4. Isolate polluted/contaminated areas/populations

5. Institute remediation procedures

Note that Education plays a big role in avoiding both individual and environmental exposures as well as in minimizing dosages. Foreknowlege allows comparative risk evaluations to be made. Regulations help codify the knowledge base and provide guidelines for toxicant handling procedures. They externalize comparative risk evaluations. Education also allows formulation of appropriate response information and the description of pertinent treatments to handle detoxication.

What about treatments in the clinical sense?

Procedure usually involves the following steps:

1. Take a complete history of the intoxication (acute, chronic, contributing conditions)

2. Evaluate the subject's condition, identify the poison and dose, if possible

3. From databases or references determine the expected pharmacokinetics for clearance (How long will elimination take and what effects can be expected in the dose range involved?)

4. If the intoxication is acute and recent, act to decrease the dosage.

5. If minimal risks are involved, provide medical support with minimal interventions.

6. If there is a potential for extensive morbidity or mortality undertake active interventions:

* For oral exposure: dilute the stomach contents, pump out the stomach, give one or more cycles of activated charcoal, provide antidotes or antagonists as appropriate.

* For blood exposure: provide chelators or appropriate antibodies, or perform dialysis or transfusion as required.

* Since dermal and respiratory exposures normally pass through the blood the only palliative measures that apply beyond the treatments suggested for blood exposures are removing the subject from the exposure source and doing a thorough washing with appropriate detergents of the exposed dermal surfaces.